A 40-fs time-resolved absorption study on cis-stilbene in solution: observation of wavepacket motion on the reactive excited state

Time-resolved absorption spectroscopy with ∼40 fs time resolution was applied to the S1 state of cis-stilbene in solution. An oscillatory signal caused by ∼220 cm−1 wavepacket motion was clearly observed. Its damping rate was much faster than the isomerization rate and insensitive to the change of the solvent. The obtained ultrafast pump–probe data showed that the dephasing of the initial coherent wavepacket motion first takes place rapidly, followed by the ∼1 ps isomerization reaction. The significance of the ∼220 cm−1 mode, intramolecular vibrational energy redistribution (IVR) process, and high anharmonicity of the S1 potential surface in the initial stage of cis-stilbene photochemistry is discussed.